1. Field of the Invention
The present invention relates to an optical system including two or more lens units, which is suitable for, for example, image pickup.
2. Description of the Related Art
Various aberrations such as chromatic aberration which vary with focusing are often corrected by using extraordinary dispersion materials. In particular, macro lenses whose maximum image-forming magnification is large and aberration variation with focusing is significant are formed by using extraordinary dispersion materials having a low dispersion to suppress the variation of the chromatic aberration. Moreover, it is also possible to suppress such variation of the chromatic aberration by using high dispersion resin materials having extraordinary dispersion characteristics.
For example, Japanese Patent Laid-Open No. 2003-329924 discloses a macro lens that suppresses variation of various aberrations with focusing by using a low dispersion material. Moreover, Japanese Patent Laid-Open No. 2009-15026 discloses a macro lens that suppresses variation of various aberrations with focusing by using a high dispersion resin material having extraordinary dispersion characteristics.
However, the low dispersion material and the high dispersion resin material having the extraordinary dispersion characteristics used for the macro lenses disclosed in Japanese Patent Laid-Open Nos. 2003-329924 and 2009-15026 generally have a low refractive index, which makes it difficult to suppress variation of spherical aberration with the focusing. On the other hand, providing a large refractive index to a low dispersion material in order to correct the chromatic aberration may increase a volume and a weight of the material.
In addition, such high dispersion resin materials generally have characteristics that are easily changed with temperature, humidity and ultraviolet, which causes concern about environment resistance.